Polarity-driven oxygen vacancy formation in ultrathin LaNiO3 films on SrTiO3

Oxide heterostructures offer a pathway to control emergent phases in complex oxides, but their creation often leads to boundaries that have a polar discontinuity. In order to fabricate atomic-scale arrangements of dissimilar materials, we need a clear understanding of the pathways by which materials resolve polarity issues. By examining the real-time lattice structure in-situ during growth for the case of polar LaNiO3 synthesized on non-polar SrTiO3 (001), we demonstrate how films in ultra-thin limit form as LaNiO2.5 and then evolve into LaNiO3 as the thickness increases. Theory explains how the polar energetics drives the formation of oxygen vacancies and the stability of these phases with thickness and structure.